One of the rigs crew members thought
he was isolating the airline he was
using and unintentionally closed the air
supply to the Diverter overshot packer.
With the lack of constant pressure
within the packer the element began to relax and therefore allowed the mud to escape between the Bell nipple and overshot.
The estimated amount lost was 8 BBLS of WBM and was contained on the platform and cleaned up.